Telephone system



J. E. OSTLINE ET AL TELEPHONE SYSTEM July 30, 1929. 1,722,539

Filed Dec. 7, 1925 3 Sheets-Sheet 1 1 l l l l h 1H Inf-mm Jul-m E. UsTZms lsuuifi'uzrmas TTEdmunds v July 30, 1929. .1. E. OSTLINE ET AL TELEPHONE SYSTEM Filed Dec. '7, 1925 3 Sheets-Sheet 2 kkm huren DPS Julm E. ULTTZma-and-Thumas PT. Edmunds y 1929, J. E. OSTLINE Ef AL 1,722,539

TELEPHONE SYSTEM 1 Filed Dec. 7, 1925 3 Sheets-Sheet 3 Fig: 5

Patented July 30, 1929.

' UNITED 'sTA's 1,722, ,-EN'T oFF'ICE JOHN E. osrLmE, or LIVERPQGL, Arno rrioMAs M. VEDMUNDS, or Kines Limit, ENG- 'VLAND, ASSIGNORS, BY'MESNE ASSIGNMENTS, TO AnroM'A'rIo ELECTRIC me, or CHICAGO,ILLINOIS,-A CORPORATION OFIIDELAWARE.

TELEPHONE SYSTEM.

Application filed December 7, 1925, Serial No.

This invention relates to telephone systems and is particularly concerned with improved circuit arrangementsfor automatic switchessponds to impulses dialled bya calling-party is prevented from operating-when all the lines of a group are busy should impulses be transmitted at this time.

Another feature of the invention concerns an improved arrangement for connecting battery potential to a speaklng conductor to indicate the busy condition of a line or group of lines so that there is no dilierence' of potential between the speaking conductors during the transmission of a busy flash signal.

A further feature of the invention concerns improved apparatus for busy flash and tone circuits which is common to a plurality of switches, the busy flash and busy tone circults being connected to common'conductors over alarm devlces and means areln'serted 1n the common conductors so that 'a fault on any conductor can bereadily located byseparat ing said conductors. Some examples of the invention are shown in the annexed drawings, in which:

Figs. land 6 showtheimproved circuit arrangements for a Strowger selector switch.

Fig. 2 shows an improved busy flash circuit 7 using one common busy conductor per eX- change, and I v Flg. 3 shows a similar busy flash circuit using individual transformers per switch group instead ofa common conductor per 7 I -ries relay G releases and at the normally modification using one conexchange.

Fig. 4 shows a 'ductor commonto the exchange, wh le Fig. 5 shows a modification ofthearrange' I ment shown in Fig. 4 using one transformer' per group of switches.

73,55 and in Great Britain January 23, 1925.

tion with selectors such as the selector "5, Fig.6; a

A description will first begivenof the op p eration of the selector S, Fig. 1'."

As soon as the selector S is taken into use by a calling party, line relay'A energizes over the following circuit battery, 7 upper -'wi nd-' mg of relayA back contacts and armature D of switch relay D, negat ve line conductor,

over the calling partys loop,'positive linefcoii ductor, armature D of switching relayD and its back contact, lower winding of relay A,

eleventh cam spring Y and its back contact, common conductor 41, secondary winding of' the dial tonetransformer DTto earth. Re lay A upon energizing completes at A a cir-.

cuit for slow release relay B. Relay B upon energizing earths the release trunk conductor P at B and at B? prepares a circuit for-the vertical magnet VM. Relay A responds to impulses generated by the calling party and at the back contact of A repeats'them to the vertical magnet VM over the circuit: earth, A D B throughthe slow series relay '0 to vertical magnet VM and battery. Relay G energizes in series with the vertical magnet and being slow. to release remains operated" throughout the'series oflmpulses, and MC opens one point in the circuit of rotary magnet BM and prepares a circuit for stepping relay E. v

' By the operation of the vertical magnet VM, the wipers 1, 2, and 3 of the selector S are raised opposite to the desired set of bank contacts. Upon the first vertical step, the off normal contacts ON and 0N close. 1 By the closure of contacts ON the circuit, from earth,

B C ON, Efto battery, is completed,"

therebyoperating thestepping relayF. Re-

1 lay E uponen'ergising prepares a clrcuit at E for rotarymagnet RM and at E completes a locking circuit for itself. The ofl? normal contacts 0N prepare a circuit for the release magnet- RM At the end mitted to the vertical magn'et'VM, slow seclosed contacts controlled by C completes of the series of impulses trans a a circuit for the rotarymagnet RM. By the operation of.{ the rotary magnet RM, the

wipers 1, 2, and 3, arerotated in search of and Open the locking circuit for" relay "E a free trunk; The interrupter springs R break'at the completion of each rotarystep stepping relay E over ofl normal contacts ON, rotary interrupter springs R winding of relayD tothe earthed test conductor P. Switching relay D upon energizing, disconnects the windings of line relay A from the line conductors at D and D ,.and at U connects the test wiper 3 to the test conductor P,

. and atD? opens a point in the verticalmagnet circuit. Shortly after the de-energization of line relay -A, releases relay Ede-energizes fee and closes at B a point in the circuit for the releaseinagnet RM 7 The release. of the connection is brought about by the removal of guarding potential from the conductor engaged by test wiper 3 which results in the de energization of switching relay D. Relay D upon de-energizing completes at D a circuit for the re lease magnet which includes the back contact of A back contact of D", back contact of B and oil, normal. springs 0N Assuming now that all the lines in the se- 1 lected level arebusy, the eleventh step cam springs V, W, X, Yand Z will be operated on the eleventh rotary step. The Z springs complete the following circuit for the busy relay BR, Fig. 2:,earthed test conductor P,

1 cam'spring Z and its front contact, common conductor 6, busy relay BR (which is com-- .mon to] a group of selectors), busy cam springs BUS (which are common to the exchange) to battery.

Thecam springs Y connect the lower winding of relay A tothe common conductor .5

whichis intermittently connected alternately tobattery through relay R and to earth throughthe low-resistance winding of the i busy ftone transformer ET by the busy cam spr ngs BCS. The cam springs V and W short, c rcuit the springsD and D of the switching relay 1) 'to maintain an energizing circuit forrelay vA, while the cam springs X ,;connects earth to the conductor 7.

- ;8 to hold relay B. energized independently testlwi-per 3, itwwill be seen that V Bythe operation of thecommon busy relay BR, the conductors 7.and 8 which are individual to the selectors are connected to gether and earth is extendedover conductor of the contacts A ofthe line relay Returning now'to the position where earth, is encountered on the tenth bank contact by "potential completes a circuit for stepping relay E over the interrupter contactsR and that relay E connects eartl to' thetest wiper 3st E", and alsothat at E a circuit closed this earth.

for the rotary magnet RM to move the wipers 3 into engagement with the eleventh contact P to which the over-flow meter OFM is connected. The earth connected up at E will operate the meter OFM before the rotary interrupter springs R open asthese's'prings are arranged to open at the end of the rotary step and the meter will record the numbero'f times the selector S finds all the lines of a group busy. The opening of the interrupter springsR will cause relay E to de-energize and the removal of earth at IE will allow 7 the switching relay D to energize inseries with relay E, Relay Ehowever, is not energized in this circuit on account of the high resistance of the relay'D. Relay D, upon energizing, will operate its armatures D D D and D. The operationrof the armatures D and D will have no efi'ect on line relay A as these contacts are short circuited by the cam springs V and W. The armature D disconnects the test wiper 3 from one side of the switching relay-D, whilearmature D magnet VM soth'at in case the calling party should dial when all the lines of a group are busy, the vertical magnet VM will not be operated.

If the calling subscriber is at an automatic substation, the busy tone current which is connected to the positive speaking conductor 'opensa point in the circuit of the vertical over the circuit: earth, secondary winding of busy tone transformer BT, busy cam springs BUS, common conductor 5, eleventh cam springs Y, lower winding of relay A, eleventh cam spring W to the positive talking conductor, will indicate that the wanted line is busy. 7 I

The calling subscriber upon hearing the busy tone will replace lIIS recelver and bring about the de-energization of relay A. Relay A will open one point in the: circuit of relay B at A and when the busy cam springs 1308 are operated to disconnect battery from relay BR the latter relay will de-energize and open the locking circuit for relay B for a period long enough to allow relay B to release. Relay B, upon releasing, will open the circuit of the switching relay D at B and switching re lay D will complete a circuit for the release magnet at D Ifthe callingparty is an operator at a manual exchange, the battery through relay 1% which is intermittently connected to the positive talking conductor will flash a supervisory signal at the 'operators position over thecircuit: battery, relay R, busy cam springs BUS, common conductor'5, eleventh step cam springs Y, lower winding of relay A, eleventh step cam spring W to the positive speaking conductor.

Referring now to Figure 3, a busy tone and busy flash circuit is shown using a transformer for each group of switches instead of i one busy common per exchange as in Fig. 2. This arrangement has theadvantage that in the event of conductor 5 being permanently earthed, it would not render the whole of the exchange busy flash arrangements inoperative. The relay-BB Fig. 3, corresponds to the relay BR, Fig. 2, and the conductors 4 5 6 7 and 8 to the conductors 4, 5, 6, 7 and 8, respectively, of Figure 2. The additional equipment in Fig. 3 comprises the busyfia'sh transformer BFT and the three springs 10 on relay BB One side of the primary winding of the busy flash transformer BFT is connected to earth direct and the other side to earth through the low resistance winding of busy tone transformer BT which is common to the exchange. The secondary of the transformer is earthed at its middle pointQ One secondary winding is common to one group of switches and the other secondary winding is common to another group. The contact springs 10of relay BB alternately connect up busytone and battery to common conductor 5 as relay BB is intermittently operated by the interrupter cam TC which is common to the exchange.

' Referring now to Fig. 4 an alternative busy flash and busy tone arrangement is sh own together with the line and release relays of the selector switch. The line relay A and release relay B correspond to the relays A and B of the selector S and the eleventh step cam springs Y and X correspond to the eleventh step cam springs Y and X, while the busy cam springs-B08 correspond to the springs BCS, Figure 2, and as before are common to the exchange. A busy tone transformer ET is provided which is also common to the exchange. It will be seen that the release relay B is provided with two windings. The lowor winding is a locking winding which is energized over the eleventh cam springs X while the eleventh cam springs Y connect the lower winding of the line relay A to the common busy conductor which is alternately connected to battery and busy tone by the cam springs BCS The auxiliary winding on relay B is to prevent the relay from being looked up over the common conductor 12.

It will be noted that withthe arrangement shownin Fig. 4 the conductors 5 6 and 7 and the relayBR are not required and moreover also.

Referring now to Figure 5, an alternative busy flash and busy tone arrangement is shown which is similar to Fig. 4, except that a busy tone transformer BT is provided for each group of switches instead of one transformer per exchange. The eleventh step cam springsY and X are connected up as in Figure 1, and a busy relay BB is provided which is controlled by the busy cam springs BCS The transformer ET is provided with a split secondary winding similar to the, transformer BFT, Figure 3.

A description will now be given of the selector S Figure 6, in conjunctionwith the busy flash and busy tone apparatus shown in Figure7. ;x Referring now to Figure 7, the apparatus below the dot and dashline'com'prising the dial tone machine DT earth alarm relay EAR, common interrupter C and the common busy tone transformer CBT is common to the whole exchange. The apparatus shown abovethedot and dash line comprising the alarm. relay EAR? the busy relay BR? and the busy tone transformertBT are' common to a group of selectorsof the type shown at S Fig. 6. i y

When the selector S is taken into use the line relay ARenergizes over the circuit bating conductor, calling loop, speaking con ductor 23, 24, lower winding of AR, 11th step cam spring-25 and its resting contact, dial tone conductor DT, secondary windingof dial tone machine DT 'to earth. AR upon energizing, completes the follow ing circuit for release'relay BR earth, 28, 29, 30, lower winding of relay BR'tobattery. Release relay BR, upon energizing, earths the-release trunk RT at 32 and at 33ifurther 8O tery, upper winding of AR, 21, 20, speak- Relay opens the circuit-of release magnet Z. Be-

PW, W and W are: raised step by step" and come to rest opposite a level of bank con"- tacts corresponding to the impulses received. Relay GR energizes in series with the vertical magnet V and'being slow toreleaseremains energized throughout the vertical movement.

At armature 36, relay CB closes in conjunction with off-normal contacts N, whichclose v on the first vertical step, a circuit fromearthed release trunk RT for stepping relay ER; Stepping relay ER accordingly'energizes and; closes a locking circuit for itself at armature this rotary magnet R. p 7' 4 t the end of the vertical movement, slow relay OR 'de-energize's and closes the rotary magnet circuitat contacts 3738; Rotary magnet R now energizes and advances the wipers PW, -.VV and W into. engagement I with the first set of bank contacts'in' the selected level, and, near the end of its stroke, opens at interrupter contacts 41 the circuit of stepping relay ER, whereupon stepping relay ER de-energizes'and opens the circuit of rotary magnet R at armature 39. -Rotary magnet R now de-energizes and closes ts in-.

terrupter contacts 41' again.

The further operation now depends upon I whether the trunk terminating in thefir'st set of bank contacts is busy or idle. the trunk is idle, switching relay DB energize's, seizing the trunk. Assuming onthe In case 7 y 110 A 40 and at'armature 39 prepares a circuit for r other hand that the trunk is busy, switching relay DR being short circuited by the earth potential encountered on the busy test contact by test wiper PW, does not energize, and the gagement with the idle trunk. The switching relay DR, being no longer short circuited,

energizes over the following circuit: earth, 32, RT,-DR, normally closed 11th step cam springs controlled by spring 27,41, N, to battery. Stepping relay ER does not energize in this circuit on account of thehigh re sistance of sw tching relay DR. -Upon ener gizing switching relay DR disconnects earth from armature 29 of line relay AR at armature 28, thereby opening the circuit of release relay BR; opens the test circuit and prepares the holding circuit at armature 43; and at armatures and 23 disconnects the incoming line conductors from line relay AR and extends them by way of wipers W and W to the line relayof the next switch The line and releaserelays ofthe next switch now energize and prepare the switch for operation 'in' the usual manner, feeding earth potential back over wiper PW beforethe-slow release relay BR falls away.-

The release of the'switch S 1 is brought about bythe calling party hangingup his receiver and thereby causing the final selector in use to release and. remove earth from the release trunk RT. The removal of earth from the releasetrunk RT brings about thede-energization of switching relay' DR. Relay DR upon de-energizing completesat armature 28 the following circuit for release magnet Z; earth, 28,29, 31, 33, 35, N Z to battery. Magnet Z restoresthe switch to normal position,-the energizing cir- V cuitfor magnet Z being opened by the otf normal contacts N when the switch shaft is in normal position. I

Returnmgnow to where the rotary magnet R is hunting for an idle line and assuming that all the lines of the selected group are busy, magnet R will advance thetest wiper PW into engagement with'the 11th step contact 45, and the and wipers to an open posit-ion. When the rotary magnet R makes the 11th step the 11th step cam springs 25, V

26 and 27 are operated.

By the operation of cam springs 27 switching relay DR is short circuited. The momen tary' connection of earth potential to conductor 46 will not cause the operationrof relay ER as the interrupter contacts 41 are opened at this time. The operation of spring disconnects the lower winding of line relay AR from earth via the dial tone conductor DT and connects it tothe busy common conductor BC which is intermittently connected to earth and battery alternately. The operation of spring 26'prepares a circuit for the upper winding of release relayBR, and this circuitincludes the hold common lead HC.

Referring now to Fig. 7 it will be seen that the busy relay BR is. intermittently ener gized and de-energized'by the common interrupter C which is arranged to connect batteryand earth alternately to relay BR The battery and earth potentials are arranged to be connected for 0.75 of a second and disconnected for 0.7 50f a second.

Assuming that the, common. interrupter springs are in the position shown batterypotential will be connected tothe busycommon conductor BC through the alarm fuse /,'up-

per low resistance winding of the alarm re lay EAR armature 52, armature 60, testing link Z Battery potential will also be C011"? nected to the hold common lead 'HC, over armature 61 and testing link Z. i

The battery potential on conductor BC is connected to'thepositive speaking conductor over the following circuit and serves to flash a supervisory signallwhen' the-calling party is an operator; battery, fuse 7", upper winding of relay EAR 52, 60,5, conductor BC,

working contact and cam spring 25, lower winding of line. relay AR, 24, 23, speaking conductor.

The substitution of battery potential for earth potential on the lower winding of relay AR will cause the'latter to fall away and open the circuit of release relay BR. Relay- BR being slow to release'is now maintained energized by its upper winding over the following circuit battery, fuse 7, contact 61 of BR lead HC, upper winding of RR, working contact and cam spring 26, 34, 33, 31,

29,28, to earth. It will be seen that battery I potential is connected to the and speak ing conductors through the upper and" lower windings of line relay AR and. since these windings-are alike there will be no difference of potential between the speaking conductors during the busy flash period. When the common interrupter springs are operated relay BR will be short circuited by the'battery potential through relay EAR and the hold common lead HC will be disconnected and'the busy' common lead BC. connected to earth. through the secondary winding of the busy tone transformer RT. The line re-Q lay AR will not be de-energized under these conditions and release relay BR will be main tained operated over its lower winding. A busy tone current will now be connected to the speaking conductor over the following circuit: secondary winding of busy tone transformer 3T normally closed left hand armature of relay BB conductor BC, working contact and cam spring 25, lower winding of line relay AR, 24, and 23 to the speaking conductor. The calling party in this case will therefore receive a busy tone signal. The release of the switch S in either case is brought about by the calling party opening and 'EAR are provided to give an alarm in case the common conductors 1C,- BC or HG should becomeearthed. It will bereadily seen that an earth on conductor ICwill bring aboutthe energization of relay EAR which at contacts 50 completes any suitable alarm circuit. This fault can be readily located by removing the links Z and Z An earth on conductor BC will cause the fuse f to blow and complete an alarm circuit in known manner. An earth on the speaking conductor will not cause the fuse f to blow, but instead the alarm relay EAR will operate over its upper winding and lock up over both windings and complete at 51- a circuit for lamp L and alarm relay EAR The exchange attendant seeing the lamp L glowin can locate the fault byremoving the links Z to Z An earth on the common lead HG will-cause the fusef to blow and complete an alarm circuit, this fault. can be readily. located by removing the links Zto Z Should the 'common busy tone lead-BT vbecome connected with battery potential the ruse f will blow and complete an alarm-circuit, this fault being readily located: by 'rek moving the links Z and Z --.Having' now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, wedeclare what we claim is 1. In a telephone system, a selector switch having aline relay controlled over an incoming talking conductor, a primary magnet controlled by the line relay, a secondary magnet-for operating the switch to select an idle trunk, and means controlled by the secondary magnet and independentiof the net controlled by the line relay for selecting line relay forpreventing the line relay from energizing'the primary magnet, while maintaining the control 'of therelay over -'the incomingtalking conductor. y a selector. switch a group of trunks, a secondary magnet, and means controlled by the'-secondary;magnet responsive to a busy group of trunks being selected by the primary magnet for preventing the line relay from energizing the pri: mary magnet while maintaining the control of the relay over theincoming talking conductor. f 1

3. In a telephone system-,a plurality'of automatic switch es, each Switch having two incoming talkingconductors and aline re lay, a source of potential connectedto' both incoming talking conductors of each switch throughthe Winding of the line relayofthe switch, means common to all of the switches and effective upon the occurrence of predetermined conditionsin any of theswitches V for periodically connecting one of thetalk' ingc'onductors of-the switch in whichv those conditions occur to apoint at the's'ame' 'po tentialas that of the other conductor. I 4. In a telephone system, a plurali'tyof incoming talking conductors and" a,--linerelay, a source of potential connected to'both incoming talking conductors of each switch termined'conditionsin any of the switches:

for periodically connecting one of the talk automatic switches, each switch having two ing conductorsof the switch in which those I conditions occur to a-point 'at the same'pov tential as that of the other conductor, 1 said vconnection. being made through a win'ding on the line relay of the switch inwhichthose conditions-occurred. I 5. In a telephone-system,

incoming talking 'conductors and a line relay, a source'of potential connected tol-the the winding 'oftthe line relay of-the switch,

a' plurality of automatic switches, eachsWitchhav-ing two '1 v incoming conductors offea'ch switch through each switch 'havingaccessible-thereto a plu rality'of groups of trunks, means cannon: to all of the switches.and"efie'ctivewhen any switch encounters a busy group F of trunks for periodically ductors of the switchthat encounters}; the busy group of trunks to-a'" pointfat the same connecting one of the con,-

incoming talking conductorsiand a line relay, a source ofupotent-ial connected to -the incoming conductors of each switch through the winding of the line relay; of the switch,

each switch having accessible-thereto; aQp'lurality' of {groups of (trunks; I means -common to all 1 of, the switches and'efi'ective; when any switch encounters abusygroup of trunksfor periodically conn'eotin'g one of'the' condiictors of the switch of trunks to a point at the same potential as that encounters the lousy group 1 iio list potential as s the other conductor of that 1 tion being made through a winding of the inerela, is

1 85 In a telephone system, a plurality of groupsof trunks, a switch having two incomlng talking conductors and a line relay, andhaving accessto the groups of trunks, means for operating .theline relay when theinconr ing' conductors are seized, a V guarding relay, meanscontrolled by the release relay for marking the switch busy, said release relay beingoontrolled by the line relay, and addltional .means eflective if a busy group of relay v trunks is encountered to control said release 9. In "a telephonesystem,a selector having fa relay, a release relay, and a primary .magnet, cincuitcon-nections controlled byv the line relay for controlling'the primary magnet, and means effective under predetermined conditions for substituting a winding of the release relay for the primary magnet in said circuit.

= .10. In a telephone system, as'elector having a line relay, a release relay, and-a primary magnet, circuit connection-s controlled by the linerelay for controlling the-primary-magnet, and means eifectivc responsive to a busy 1 group ottrunks being encountered for sub 7 I stitutinga winding of the release relay for Y primary-magnet intsaidcircuit.

.11. In a -telephone system, a plurality of switches, each .switch including-1a. line relay and a release relay, said guarding irelayfhavmg, two operatingwindings, one of which windings is controlled by the line relay in a circuit local to the zswitchjand the otherconover, a'circui't common to allot the switches.

12. In a common @to the switches, said apparatus including alarm devices through: which the signals :a-re transmitted, andmeansincluding readily detaehableiconnecting links, whereby 'alny defective group of switches may be readi-ly alisconnectedifrom the apparatus.

13. In 18; telephone system,- a plurality of switches having an incoming stalking Yeantluctor, means common-to all otthe switches for applyingsgs'ignal-ling current v to a conductor of any of the switches,

ncluding low resistance-alarm coming talking conductors of the V telephone system, a plurality of 3 groups of switches, bnsy gsign aglling appara 19. In a-telephone-system, asel having a line relay controlled rower an'ixncom-g conductonaprimarymnagnet con,- a

:ing talking trolled bythe line relay for selecting a :gronp relay through which the signalling current is sent, and means controlled by said relayfor' inserting a high resistance.in'serieswith the low resistance. V 145 In a'telephone system, a plurality of groups of switches, each switch having a pair of incoming talking conductors, a relay de-,

'viceindividual to each group of switches,

means'common to all of said devices for intermittently energizing and deenergizing said devices, and means controlled by each of said devices; for supplying busy flash current to all of the-switches of its group,

15. In a telephone system, a plurality-of groups of switches, each switch having a pair of incomingtalking conductors, a relay clevice individualto each group of switches,

meanscommon to all ofsaid devices for intermittently energizing and, deenergizing said devices,;mean-s controlled by each of said devicesiorsupplyingbusy flash current to all of theswitches of its group, and means incach switchieifective under predetermined conditions to transmit said busy flash current over. one of its incoming talking conductors;

.16. ,In. a telephone system, a plurality of automatic switches, each switchhavin-g two incoming talking conductors and a line relay, each switch having accessible thereto a plurality of groups of trunks, means common to all of the switches and effective responsive to I any one switch encountering a busy group of trunks for signalling over at least one of the mcomlng talking conductors of that one switch, saidymeans. including a circuit ar rangement whereby. both of the talking conductors of the switcharemaintained at the same electrical potentiatduring signalling. V -17. In a telephone ,system,a plurality of automatic .switches, each switch having two incoming talking conductors and a line relay,

tivefupon the occurrence of predetermined conditions in any ofzthelswitches forperiodig callysignalling over atleast one of said inswitch' which the conditionsoccurred, "saidmeansim eluding circuit arrangements for maintaining both-of said incoming talking conductors at the samexelectri'cal potential dnr-ing'flsigznal ling. 1;"

I18. In a telephone system, avzswitch two incomingsta-lking conductors, a line nelay controlled over sa-id talking. conductors, means at, the switch for signalling, over said incoming talking conductors, saidmieans ill-- eluding means for periodically connecting both of said stalking conductors to points of the .fsame electrical potential :tln'on'gh the winding of; thelin'e r re l'ay'.-;1

having of trunks, an automatically operatedseoond- 'means .commonto all oithe switchesand efllec- 1 I ary magnet for selecting an idle trunk in the selected group of trunks, a switching relay for disconnecting the line relay from the line, and means effective responsive to'the failure of the secondary magnet finding an idle trunk in the selected group of trunks for rendering the switching relay inoperative to disconnect the line relay and for preventing the line relay from operating the primary magnet.

20. In a telephone system, a selector switch having a line relay controlled over an incoming talking conductor, a primary magnet controlled by the line relay for selecting a group of trunks, an automatically operated secondary magnet for selecting an idle trunk in the selected group of trunks, a switching relay for disconnecting the line relay from the line, means effective responsive to the failure of the secondary magnet finding an idle trunk in the selected group of trunks for rendering the switching relay inoperative'to disconnect the line relay, for preventing the line relay from operating the primary magnet, and for connecting a source of signalling current to an incoming talking conductor. 7 I

21. In a telephone system, a selector switch having a line relay normally connected to an incoming talking conductor, a primary magnet controlled bythe line relay for causing the switch to select a group of trunks, means for then causing'the switch to hunt for an idle trunk in the selected group, a switching relay for disconnecting the line relay from the talking conductor, means operative upon the switchis failure'to findan idle trunk for for the selector controlled by a flow of current over the talking conductors terminating v in the selector, a release relay responsive to the operation of the line relay, andmeans operative upon the selectors finding no idle trunkin the group for stopping the flow of current over the talking conductors, and for maintaining the release'relay operated independent of the line relay.

23. In a telephone system, a selector for selecting an idle trunk of a group, a line relay 5 for'the selector controlled by a flow'of current over the talking conductors terminating in the selector, a release relay responsive to the operation of the line relay, and means operative upon the selectors finding no idle trunk for periodically stopping the flow of current over the talking conductors, and for maintaining the release relay operated inde-- dependent of the line relay during each pe-v riod the current over the talking conductors is stopped by the last said means.

In witness whereof, I hereunto subscribe my namethis23rd day of October, A. D. 1925.

7 JOHN E. OSTLINE. In witness whereof, I hereunto subscribe my name this 28rd day of October, A. D. 1925.

THOMAS M. EDMUNDS. 

